In a liquid chromatograph mass spectrometer (LC-MS) using a mass spectrometer as the detector for a liquid chromatograph (LC), an ion source which employs an atmospheric pressure ionization method, such as an electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI), is used in order to ionize a liquid sample eluted from a column of the LC while gasifying the same sample. For example, in an ESI ion source, a high voltage (e.g. ±several kV) having the same polarity as the ion to be analyzed needs to be applied to the tip of the nozzle from which the liquid sample is sprayed.
In such a mass spectrometer, when the detection of a positive ion and that of a negative ion are alternated with a short cycle of time, the polarity of the applied high voltage needs to be switched according to the polarity of the ion. For this operation, a high-voltage power supply device configured to be capable of switching the polarity of one system of output voltage is used. A conventionally and commonly known high-voltage power supply device for switching the high voltages having opposite polarities is one which uses a high-voltage-resistant reed relay (for example, see Patent Literature 1). However, the switching speed of the high-voltage power supply device using a high-voltage-resistant reed relay is slow since the switching of the polarity of the output voltage is achieved by the mechanical switching of a contact point.
To address this problem, the present inventor has proposed a high-voltage power supply device having a novel configuration as described in Patent Literature 2. This high-voltage power supply device has a positive voltage generation circuit and negative voltage generation circuit, each of which includes a DC-DC conversion circuit using an isolation transformer, with one resistor parallel-connected to the positive output terminals formed by the positive voltage generation circuit and another resistor parallel-connected to the negative output terminals formed by the negative voltage generation circuit, and the two resistors connected to each other in series. Using one end of the series-connected resistors as the reference end, a high positive or negative voltage is extracted from the other end.
In this high-voltage power supply device, the high voltage is generated in each of the positive and negative voltage generation circuits by supplying a predetermined drive signal from a control circuit to a switching element included in each of the positive and negative voltage generation circuits. When the high-voltage output is switched between the positive and negative polarities, the positive and negative voltage generation circuits are controlled so that the output voltage of one circuit changes from a high positive voltage +HV to zero while that of the other circuit simultaneously begins to change from zero to eventually converge to a high negative voltage −HV after overshooting this level. This intentional overshooting of the voltage which is being raised from zero is aimed at reducing the amount of time required for the positive/negative switching of the polarity.
In order to additionally increase the speed of the positive/negative switching of the polarity, the present inventor has further improved the high-voltage power supply device described in Patent Literature 2 and proposed a novel high-voltage power supply device in Patent Literature 3. In this high-voltage power supply device, the resistors respectively connected in parallel to the positive output terminals formed by the positive voltage generation circuit and the negative output terminals formed by the negative voltage generation circuit in the previously described device are replaced by switch circuits consisting of FETs or similar devices. A circuit consisting of two resistors connected in series is connected in parallel to each of the switch circuits. A signal extracted from the connection point of these two resistors on the positive side is sent as the on/off drive control signal to the negative-side switch circuit, while a signal extracted from the connection point of the two resistors on the negative side is conversely sent as the on/off drive control signal to the positive-side switch circuit.
In this high-voltage power supply device, for example, consider the situation where the positive voltage generation circuit is operated to make the high positive voltage +HV appear between its output terminals, while the negative voltage generation circuit is in effect inactive, with the voltage between its output terminals at approximately zero volts. In this situation, a voltage which equals the high voltage +HV divided by the ratio between the resistance values of the two resistors at the positive output terminals appears at the connection point of these two resistors and given to the switch circuit on the negative side. Consequently, this switch circuit turns on, and the conduction state is established between the output terminals of the negative voltage generation circuit. From this state, when the polarity of the voltage is switched from positive to negative, the positive voltage generation circuit is deactivated, while the negative voltage generation circuit is activated. When the voltage between the output terminals of the positive voltage generation circuit decreases to a predetermined level, the switch circuit on the negative side turns off. Meanwhile, the voltage between the output terminals of the negative voltage generation circuit increases, and this time, the switch circuit on the positive side turns on. Consequently, the electric charges remaining at the output terminals of the positive voltage generation circuit are rapidly discharged through the switch circuit, so that the output voltage of the positive voltage generation circuit rapidly decreases to zero.
In this manner, when the positive/negative switching of the polarity is performed, the two switch circuits respectively provided at the positive and negative output terminals function so as to induce the forced discharging of the electric charges remaining at the output terminals on the side corresponding to the polarity which is about to change to zero. This leads to a quick decrease in the voltage which is about to change to zero, so that the positive/negative switching of the polarity can be more quickly achieved.
For example, if the previously described high-voltage power supply device capable of the high-speed positive/negative switching of the polarity is used for an ESI ion source, it is possible to perform an LC/MS analysis while alternately switching a positive ion measurement mode and negative ion measurement mode with a short cycle of time. This is extremely useful, in particular, for a simultaneous multi-component analysis or similar analyses since both the compounds which easily turn into positive ions and the compounds which easily turn into negative ions can be exhaustively detected.
In recent years, an even greater improvement in the sensitivity of mass spectrometers has been demanded, in particular, for such purposes as the quantitative determination of trace amounts of components. Conventionally, it has not been a common practice to finely control the value itself of the voltage applied to the nozzle in an ESI ion source. However, it is commonly known that appropriately adjusting the voltage applied to the nozzle according to such factors as the properties of the target compound improves the ionization efficiency and provides a higher level of ion detection sensitivity than applying a fixed level of voltage. Accordingly, attempts have been made to improve the detection sensitivity for each individual compound by adjusting the voltage applied to the nozzle; for example, in the positive ion measurement mode, the voltage is adjusted within an approximate range of +2 kV to +5 kV according to the kind of compound, or for each ion designated as the SIM (selected ion monitoring) measurement target for each compound or each transition (combination of a precursor ion and product ion) designated as the MRM (multiple ion reaction monitoring) measurement target for each compound.
However, in an SIM or MRM measurement, it is normally necessary to change the ion or transition as the measurement target within a short period of time of a few milliseconds to several tens of milliseconds. Attempting to change the value of the voltage applied to the nozzle within such a short period of time causes the following problem.
Not only the aforementioned high-voltage power supply device but also any high-voltage power supply device of the same type normally has a capacitive load located at its output terminal to smooth the output voltage. In the phase of increasing the voltage value (absolute value of the voltage), the voltage rises at high speeds since the capacitive load can be quickly charged by increasing the output current. By comparison, in the phase of decreasing the voltage value (absolute value of the voltage), the electric charges accumulated in the capacitive load need to be discharged through a channel which includes output resistors and other elements. Therefore, the speed of decrease in the voltage is considerably lower than that of the increase in the voltage. For example, in one example of the high-voltage power supply device according to Patent Literature 3 manufactured by the present applicant, the process of increasing the voltage from +2 kV to +5 kV requires 1 to 5 msec, while the process of decreasing the voltage from +5 kV to +2 kV requires 10 to 50 msec, which is approximately ten times slower than in the voltage-increasing process.
For example, in such applications as a simultaneous multi-component analysis of agricultural chemicals residues, it is necessary to sequentially perform the measurement for ions originating from a plurality of target compounds using the technique of MRM measurement. If the process of changing the applied voltage requires a considerable amount of time as in the previously described case, it will be necessary to shorten the data acquisition time (or so-called “dwell time”) or limit the number of MRM transitions to be concurrently subjected to the measurement. In the former case, the detection sensitivity will be sacrificed. In the latter case, fewer compounds can be simultaneously subjected to the measurement, and in some cases it may be necessary to perform the measurement multiple times for the same sample.